The present invention relates to phase change optical information recording mediums having a recording layer of changing phase between a crystalline state and an amorphous state in accordance with the intensity of an irradiation beam, and in particular, it relates to a method for producing an optical information recording medium capable of making an initialization process unnecessary.
Recently, optical information recording mediums have been extensively studied and developed as means for recording, reading and erasing an immense quantity of information. Especially, a so-called phase change optical disk which records/erases information, using the fact that the phase of the recording layer changes reversibly between a crystalline state and an amorphous state, has the advantage that only by changing the laser beam power, old information is erased while new information is being recorded simultaneously (hereinafter referred to as xe2x80x9coverwritexe2x80x9d). Thus, such optical disk is regarded as being full of promise.
As the recording materials of such overwritable phase change optical disk, chalcogen alloys are mainly used which include Inxe2x80x94Se alloys (see xe2x80x9cAppl. Phys. Lett. Vol. 50, p. 667, 1987xe2x80x9d), Inxe2x80x94Sbxe2x80x94Te alloys (see xe2x80x9cAppl. Phys. Lett. Vol. 50, p.16, 1987xe2x80x9d), and Gexe2x80x94Texe2x80x94Sb alloys (see Japanese Patent Laid-Open Publication Sho No. 62-53886), which have a low melting point and a high absorption efficiency for a laser beam.
When information is actually recorded/erased on/from such optical disk of a chalcogen alloy, at least one kind of dielectric layer of a material selected from the group consisting of metal or semi-metal oxides, carbides, fluorides, sulfides, and nitrides is generally formed directly above and/or under the recording layer in order to prevent the substrate from being deformed due to heat produced on recording/erasing, to prevent the recording layer from being oxidized, and/or to prevent the substances from moving along the guide grooves or from being deformed.
Optical disks having a three- or four-layered structure which includes a recording layer of a chalcogen alloy, a dielectric layer provided directly under and/or above the recording layer, a reflective layer which also acts as a cooling layer (for example, Al-alloy) provided on an opposite side of a transparent substrate from the recording layer, provided on the substrate, are the mainstream of the phase change optical disks because they are preferable in terms of the recording/erasing characteristics.
In general phase change optical disks, when the recording layer is irradiated with a laser beam having a recording power to heat it up to its melting point and is then rapidly cooled, the recording layer material is produced amorphous to thereby form a recording mark. Then, when the recording layer is irradiated with a laser beam having an erasing power to be heated to more than the crystallizing temperature and then gradually cooled, the recording layer material is crystallized to thereby erase the recording mark.
Such phase change optical disks are each produced by sequentially forming thin layers as the respective layers on the substrate by sputtering/evaporation. Since the recording layer present immediately after its layer formation is amorphous, it is irradiated with a laser beam to be wholly crystallized, which is generally referred to as xe2x80x9cinitialization processxe2x80x9d, and the optical disks, thus obtained, are then shipped.
However, this initialization process takes a time of a little less than one minute to initialize the whole optical disk having a diameter of 120 mm even with the use of the most-efficient laser beam irradiation, which leads to an increase in the manufacturing cost of the optical disks. For the time required for processing one optical disk in each manufacturing substep (cycle time), the time required for the initialization process is long compared to the substrate molding step or the layer forming step. Thus, in order to eliminate a time loss taken to pass to the initialization process when the cycle time for the layer forming step is 8 seconds, the six or seven very expensive initializing devices are required. As a result, by performing the initialization process, the manufacturing cost of the optical disks is increased.
In order to reduce the time required for the initialization process, for example, Japanese Patent Laid-Open Publication Hei No. 5-342629 discloses providing an auxiliary layer of an easily crystallizable continuous film or discontinuous island-like film adjacent to the recording layer. As the components of the auxiliary layer, compounds are named which include tellurium (Te), Selenium (Se) or Texe2x80x94Se compounds.
However, according to this method, the time required for initializing the recording layer is reduced, but the initialization process cannot be eliminated as a rule, excluding the case where both of the auxiliary layer and the recording layer are comprised of extremely easily crystal-growing substances.
It is therefore an object of the present invention to provide an optical information recording medium which eliminates the necessity for the initialization process.
The present invention provides a method for producing an optical information recording medium which has on one side of a substrate a recording layer whose main components comprise germanium (Ge), antimony (Sb) and tellurium (Te) (hereinafter referred to as xe2x80x9cGexe2x80x94Texe2x80x94Sb alloy xe2x80x9d), comprising the steps of forming a crystallization assisting layer of materials having a face-centered cubic lattice system crystal structure on one side of the substrate, and forming a recording layer directly above the crystallization assisting layer. According to this method, the recording layer immediately after its formation is crystallized.
The Gexe2x80x94Texe2x80x94Sb alloys take two types of crystal phases: namely, a face-centered cubic lattice system crystal structure and a hexagonal system crystal structure. It is known that as the temperature of this alloy is raised from its amorphous state, its phase changes from a face-centered cubic lattice crystal structure to a hexagonal structure. In the present invention, the recording layer is easily crystallized when its layer is formed due to the presence of the crystallization assisting layer having the same face-centered cubic lattice system crystal structure as the recording layer.
The face-centered cubic lattice system crystal structures include face-centered cubic lattices, and face-centered tetragonal lattice; diamond-shape structures: CuAuxe2x80x94, CuPtxe2x80x94, Ni2Crxe2x80x94, Cu3Auxe2x80x94, Ni4Moxe2x80x94, Ag3Mgxe2x80x94, Ni3Vxe2x80x94, Cu3Pdxe2x80x94, and Au3Mn-type superlattices; NaClxe2x80x94, NaTlxe2x80x94, ZnSxe2x80x94, CaF2xe2x80x94, FeS2xe2x80x94, cristobalite high-temperature-, Laves phase MgCu2xe2x80x94, Cu3Auxe2x80x94, Al3Tixe2x80x94, Cu2AlMnxe2x80x94, Al2MgO4xe2x80x94, and Bi2Te3-type structures; and their interstitial and substitutional solid solutions.
The present invention also provides a method for producing an optical information recording medium having on one side of a substrate a recording layer whose main components comprise germanium (Ge), antimony (Sb) and tellurium (Te), comprising the steps of forming on one side of a substrate a crystallization assisting layer of a tellurium (Te)-free material having a crystal structure of a rhombohedral lattice system, and forming a recording layer directly over the crystallization assisting layer. According to this method, the recording layer becomes crystallized immediately after its formation.
In the present invention, the absolute value of a lattice unconformity between the crystal structure of the crystallization assisting layer and that of the recording layer is preferably not more than 8%. The lattice unconformity is represented by:
Lattice unconformity (%)=((Bxe2x88x92A)/A)xc3x97100xe2x80x83xe2x80x83(a)
A: When the recording layer is of a face-centered cubic lattice system crystal, an atomic interval in a direction  less than 110 greater than  of the crystal;
B: A particular one of the atomic intervals of a crystallized crystallization assisting layer such that the difference between A and the particular atomic interval B is minimum among the differences each between A and a respective one of the atomic intervals of the crystallized crystallization assisting layer. In the case of the face-centered cubic lattice system, it is generally the atomic interval in a direction  less than 100 greater than  or  less than 110 greater than .
When the crystal comprises two or more kinds of elements, the distance between two adjacent atoms of different kinds may be used as an atomic interval in the expression (a). When A is greatly different from B in the expression (a), the atomic interval B of the crystallization assisting layer may be assumed to be an integer or fraction times the atomic interval.
The range of lattice unconformity is preferably xe2x88x924.5 to +8%, and more preferably xe2x88x923 to +7%.
Examples of materials each of which has a crystal structure of a face-centered cubic lattice system where an absolute value of lattice unconformity between the crystal structure of that material and that of the recording layer is not more than 8% include PbTe and Bi2Te3.
Examples of tellurium-free materials each of which has a crystal structure of a rhombohedral system where an absolute value of lattice unconformity between the crystal structure of that material and that of the recording layer is not more than 8% include antimony (Sb), bismuth (Bi), antimony (Sb) compounds, and bismuth (Bi) compounds. The Sb compounds include Sb alloys, and intermetallic compounds of Sb and other metals or semimetals. The Bi compounds include Bi alloys, and intermetallic compounds of Bi and other metals or semimetals.
In the present invention, the thickness of the crystallization assisting layer is preferably not more than 200 xc3x85. If the thickness is larger than 200 xc3x85, the record erasing characteristics would be deteriorated. The thickness of the crystallization assisting layer is more preferably not more than 100 xc3x85. If this layer is excessively thin, the recording layer can be crystallized insufficiently. Thus, it is preferably not less than 1 xc3x85.
In the present invention, the crystallization assisting layer may be in the form of a continuous film or a discontinuous island-like film, which is contacted with the recording layer. Most preferably, it is a discontinuous island-like film of materials which contain bismuth (Bi) and/or a bismuth (Bi) compound.
The optical information recording medium whose recording layer is crystallized by the crystallization assisting layer provided so as to be contacted with a substrate-side surface of the recording layer eliminates the necessity for the initialization process. If a continuous film of materials which comprise bismuth (Bi) and/or a bismuth (Bi) compound is used as the crystallization assisting layer, the CNR (Carrier to Noise-Ratio) in the second or subsequent recording by overwriting is slightly lower than that in the first recording.
In comparative examples, if a discontinuous island-like film of materials containing bismuth (Bi) and/or a bismuth (Bi) compound is used as the crystallization assisted layer, the CNR in the second or subsequent recording by overwriting is substantially the same as that in the first recording.
The discontinuous island-like film is formed, for example, by sputtering such that its thickness is not more than a predetermined value.
When the crystallinity of the recording layer formed on the crystallization assisting layer is insufficient, the recording layer is preferably formed by setting the temperature of the substrate in a range of from 45xc2x0 C. through a temperature inclusive above which temperature the substrate would be deformed (at 110xc2x0 C. when the substrate is produced of polycarbonate). Thus, the recording layer is placed in a stabilized crystalline state.
The methods for maintaining the substrate at high temperatures under formation of the recording layer include (1) heating the substrate or the crystallization assisting layer which underlies the recording layer immediately before the formation of the recording layer to thereby maintain the substrate at high temperature; (2) starting to heat the substrate or the crystallization assisting layer after the formation of the recording layer has started and continuing to heat the substrate or the crystallization assisting layer during the formation of the recording layer; (3) starting to heat the substrate or the layer surface of the recording layer immediately after the recording layer has been formed; and (4) starting to form the recording layer immediately after the preceding layer has been formed, using the heat produced by the formation of the preceding layer and stored within the substrate.
The heating methods include irradiating a surface of the layer formed on the substrate (a surface of the crystallization assisting layer) with light including heat rays; and heating a substrate holder itself with a heater or the like, or using high frequency induction, flash exposure; or plasma processing.
In the producing method of the present invention, the formation of the crystallization assisting layer is preferably performed within a layer forming atmosphere to which a nitrogen gas is added.
When the producing method of the present invention includes the step of forming a first dielectric layer between the substrate and the crystallization assisting layer and/or the step of forming a second dielectric layer on an opposite side of the recording layer from the crystallization assisting layer, the formation of the first and/or second dielectric layer is preferably performed within a layer forming atmosphere to which a nitrogen gas and/or an oxygen gas is added.
The present invention also provides an optical information recording medium with a recording layer formed on one side of a substrate, the recording layer comprising materials whose main components are germanium (Ge), antimony (Sb) and tellurium (Te), wherein the recording layer is formed in a crystalline state and wherein the crystallization assisting layer is formed in contact with the substrate side surface of the recording layer, the recording layer and the crystallization assisting layer being produced by the respective above-mentioned producing methods.
In the inventive optical information recording medium, a ratio x, y, and z of the respective elements (Ge, Sb, Te) of the main components of the recording layer (Ge:Sb:Te=x:y:z where x+y+z=1) is preferably in a range shown hatched in a triangular graph of FIG. 2, which satisfies the following expressions (1)-(3) simultaneously:
0.1xe2x89xa6xxe2x89xa60.4xe2x80x83xe2x80x83(1)
0.08xe2x89xa6yxe2x80x83xe2x80x83(2)
0.45xe2x89xa6zxe2x89xa60.65xe2x80x83xe2x80x83(3)
When x less than 0.1, the optical information recording medium is not preferable in terms of stability. When x greater than 0.4, y less than 0.08, z less than 0.45 and z greater than 0.65, these conditions are unpreferable because the recording layer is difficult to crystallize.
A preferable range of the ratio x, y and z of the respective elements of the main components of the recording layer (Ge:Sb:Te=x:y:z where x+y+z=1) should satisfy the following expressions (4)-(6) simultaneously:
0.15xe2x89xa6xxe2x89xa60.3xe2x80x83xe2x80x83(4)
0.12xe2x89xa6yxe2x80x83xe2x80x83(5)
0.5xe2x89xa6zxe2x89xa60.6xe2x80x83xe2x80x83(6)
The materials of the recording layer are preferably Gexe2x80x94Texe2x80x94Sbxe2x80x94Bi alloys containing Bi in addition to Ge, Te and Sb. The materials may be Gexe2x80x94Texe2x80x94Sb or Gexe2x80x94Texe2x80x94Sbxe2x80x94Bi alloys, for example, containing hydrogen, nitrogen, oxygen, carbon, Al, Ti, Fe, Co, Ni, Cu, Zn, Ga, Se, Sn, In, Ag, Pd, Rh, Ru, Mo, Nb, Hf; Zr, Ta, W, Re, Os, Ir, Pt, Au, Tl and/or Pb. Those elements may be added from the target during the formation of the recording layer or added in a gaseous state to the atmosphere gas so as to be contained within the recording layer.
In the inventive optical information recording medium, the thickness of the recording layer is preferably 50-1000 xc3x85. If it is less than 50 xc3x85, the recording layer could not obtain a satisfactory recording sensitivity. When it exceeds 1000 xc3x85, a problem about the recording sensitivity and resolution would occur, undesirably.
The inventive optical information recording medium preferably has a 4-layered structure in which a crystallization assisting layer, a recording layer, a dielectric layer and a reflective layer are formed on the substrate in this order. More preferably, the inventive optical information recording medium has a 5-layered structure in which a first dielectric layer, a crystallization assisting layer, a recording layer, a second dielectric layer and a reflective layer are formed on the substrate in this order. The inventive optical information recording medium may further include other necessary layers additionally.
As the first and second dielectric layers, materials having high heat resistance and a melting point of not less than 1000xc2x0 C. are preferable; for example, SiO2; a mixture of ZnS and SiO2; Al2O3; AlN; and Si3N4. Although the thickness of the first dielectric layer is not especially specified, the thickness of the second dielectric layer is preferably 50-500 xc3x85. If it is less than 50 xc3x85, it can not provide a satisfactory recording sensitivity. If it exceeds 500 xc3x85, it cannot provide satisfactory overwrite cyclability. The thickness of the reflective layer is preferably not less than 300 xc3x85.
The methods of forming the respective layers include evaporation, sputtering and ion plating.
A method of confirming the presence of the crystallization assisting layer in the optical information recording medium will be described next.
The first method is to observe a cross section of the optical information recording medium, with a transmission electron microscope. The elements of the crystallization assisting layer can be specified with the aid of an electron beam diffraction apparatus and an energy dispersion X-ray analysis apparatus. When the crystallization assisting layer is island-like or very thin, it is difficult to confirm its presence, using this method.
The second method includes slowly etching layers formed on the substrate of the optical information recording medium, by sputtering, in a direction perpendicular to the substrate surface while analyzing elements present at respective positions in the layers formed on the substrate, using a secondary ion mass spectrometry (SIMS) or Auger electron spectroscopy (AES). This method is effective when the crystallization assisting layer is island-like or very thin.
According to this method, as the recording layer is slowly etched toward an interface between the recording layer and the crystallization assisting layer while the elements are being analyzed, the quantity of elements which compose the crystallization assisting layer increases toward the interface between the crystallization assisting layer and its underlying layer (generally, the dielectric layer), and after the interface is reached, rapidly decreases. By finding this phenomenon, the presence of the crystallization assisting layer will be known.
As an example, in the case of an optical disk which has a layer structure of a substrate/a first dielectric layer/a crystallization assisted layer/a recording layer/a second dielectric layer/a reflective layer/a UV set resin layer, a method of confirming the presence of the crystallization assisting layer, using the second method, will be explained as follows. First, an adhesive tape is adhered to the UV set resin layer to thereby separate the layered layer from the substrate. At this time, since the second dielectric layer is generally separated from the recording layer, the substrate on which the recording layer and the crystallization assisting layer remain is put into the secondary ion mass spectrometry or Auger electron spectroscopy to analyze the elements using by etching slowly from the recording layer side.
In the case of an optical disk where the recording layer comprises Ge, Te and Sb and where the crystallization assisting layer is in the form of a discontinuous island-like film of Bi, the presence of Ge, Te and Sb is first confirmed by analysis of the elements. As the layers on the substrate are further etched, the presence of Bi is recognized. By further etching, the quantity of Bi increases whereas the quantities of Ge, Te and Sb decrease gradually. When the dielectric layer is reached, no presence of Ge, Te, Sb and Bi is recognized. Thus, when such phenomenon is found in the second method, it can be determined that the optical disk comprises a crystallization assisting layer comprised of a discontinuous island-like film of Bi.
An optical disk whose recording layer is crystallized immediately after its formation is easily distinguished from an optical disk whose recording layer is crystallized in the initialization process, with the following method.
In the optical disk whose recording layer is crystallized by an initializing device using general laser-beam irradiation, the innermost and outermost peripheries of the recording layer are not initialized to remain amorphous state due to the composition of the initializing device. Thus, the innermost and outermost peripheries of the disk and its intermediate portion are different in reflectivity, which will be visually recognized by those skilled in the art. In comparative examples, in the optical disk whose recording layer is crystallized immediately after its formation and not subjected to the initialization process, there is no such difference in reflectivity because the whole surface of the recording layer is crystallized.